I have the following issue;
When images are uploaded via FTP after an event the interval between the 1st and subsequent images seems to be limited by the length of time taken to complete the FTP upload of the previous image (around 15 seconds in my case). The same issues is not apparent when choosing 'attached image' for e-mail notifications when 3 images are attached to the image with the minimum configured interval between the images (2s).

The hik-connect client also exhibits related but not identical behaviour, the still image displayed on the hik-connect app is typically taken a few seconds after the event.

I can reproduce this issue on 2 separate DS-2CD2T42WD's running the same firmware.